One of the most pervasive problems in motion control systems (e.g., a motor control system) is a mechanical resonance caused by a compliance in transmission between two inertias of controlled elements (e.g., motor and load). While the compliance between the motor and load is the most often cause, resonance can also come from the compliance between the motor and feedback, and sometimes come from the compliance within the load, where the load can be thought of as multiple inertias connected together by compliant couplings. Furthermore, resonance can be caused by a compliant motor mount so that the motor frame resonates within the machine frame. In other words, any of two inertias of the controlled elements coupled by compliant components may be the cause of the mechanical resonance of the motor control system.
Current autotuning methods to ameliorate the resonance problem in controlled elements provide a onetime excitation on the motor shaft and then correlate the applied torque against the measured acceleration. The relationship between the applied torque and measured acceleration yields the total inertia of the motor and load. By combining the knowledge of total inertia of the controlled elements with controller parameters of the control system, the controller gains can be set reducing the resonance effect.
While the various autotuning schemes designed according to the estimated inertias work well in the laboratory or as part of demonstration units where the mechanical coupling between motor and load is rigid, the same schemes do not work well on most practical motor control systems where the coupling between the motor and load inertia is compliant (i.e., flexible). In a motor control system with flexible coupling, the inertia estimated by the autotuning method is valid only at relatively low frequency ranges. At higher frequencies, the effective inertia of the load and motor combination varies greatly from the low-frequency (physical) inertia. The result is that the controller gains of the motor control system at higher frequency ranges, where stability problems are likely to occur, are often grossly misestimated by the resonance in the control system. As a result, when autotuning methods are applied to practical machines, the resulting controller gains for an autotuning process often produce instability.